GWT-EXT provides a API for mapping. It uses Mapstraction API to provide multiple map supports. These are the maps supported:
FreeEarthMap, GoogleMap, Map24Map, MapQuestMap, MicrosoftMap, MultiMap, OpenLayersMap, OpenStreetMap, YahooMap
Also, I am not going to do a tutorial here as there are many links available.
Check this link http://www.gwt-ext.com/forum/viewtopic.php?f=9&t=1554. It provides you a headstart on how to create Map with GWT-EXT Map API.